The Development of Negative Moist Potential Vorticity in the Stratiform Region of a Simulated Squall Line

Abstract

This paper presents evidence on the development of negative moist potential vorticity (MPV) or moist symmetric instability (MSI) in the stratiform region of a midlatitude squall line, based on a three-dimensional (3D) numerical simulation of a case that occurred on 10–11 June 1985 during the Preliminary Regional Experiment for STORM-Central (PRE-STORM). The results show that the stratiform region, though convectively stable to pure vertical displacement, is considerably unstable to slantwise displacement along the system's broad front-to-rear (FTR) saturated ascending flow. It is found that this instability evolves from boundary-layer convective instability that has previously been removed by upright convection over the leading portion of the squall system. The negative MPV in the stratiform region is mainly the result of upward and rearward transport of the low-level convectively unstable air along the sloping FTR ascending flow and of processes that reverse the signs of both the convective stability parameter (i.e., θe/z) and the absolute vorticity (v/ n −u/ s+f). The resulting symmetric instability appears to considerably enhance the vertical motion and precipitation rate in the stratiform clouds. In the stratiform region of the squall system, the negative MPV leads to a region of negative absolute vorticity or inertial instability at the upper levels, and it may be responsible for the strong anticyclonic divergent outflow in that region. Thus, the effect of the squall system is to process the low-level negative MPV in such a way as to symmetrically stabilize the lower troposphere and inertially destabilize the upper troposphere. The roles of convective, symmetric, and inertial instabilities in the development of the squall system and their implications with respect to intense oceanic storms are discussed.